1. Field of the Invention
The present invention relates to flexible polyurethane foam and a preparation process of the foam, and more specifically relates to flexible polyurethane foam which has improved durability and is frequently used for vehicle interiors, furniture cushions, bedding and miscellaneous goods, and a preparation process of the foam.
2. Related Art of the Invention
Flexible polyurethane foam (hereinafter referred to also simply as flexible foam or foam) is widely used for vehicles, furniture, bedding and miscellaneous goods due to the cushioning properties of the foam. Flexible foam is prepared by reacting aromatic polyisocyanate with polyol and/or polymer polyol containing dispersed polymer particles obtained by radical polymerization of acrylonitrile and/or styrene in the polyol, in the presence of water as a blowing agent, silicone based surfactant, catalyst such as amine and a tin compound, and, when needed, crosslinking agent, flame retardant and colorant. Water, the blowing agent, reacts with aromatic polyisocyanate and releases carbon dioxide which acts as blowing gas. At the same time, aromatic polyurea is formed and built in the foam structure.
In recent years, use of a conventional blowing agent (Freon), CFC-11 has been forbidden on the basis of Montreal treaty in order to protect global environment. Water was conventionally a blowing additive when Freon was used. However, as a result of the treaty, water has been used as an alternative blowing agent so as to exhibit similar blowing effect. Thus, amount of water used in the formulation has been increased. Furthermore, cost reduction has been strongly demanded in recent years, lower density foam is desired for weight reduction. In application to vehicles, reduction of foam density is also required for weight reduction in order to cope with fuel economy. As a countermeasure to these demands for density reduction, the amount of water used for the blowing agent is liable to further increase. Increase in water leads to greater amount of generated carbon dioxide gas and is thus effective for density reduction of the flexible foam. On the other hand, resulting aromatic polyurea impairs compression set and dynamic durability in vibration condition under load of flexible foam. Further, lowered foam density itself results in the impairment of such strain property and endurance characteristic.
Such damage of shape stability and durability of flexible foam leads to various ill conditions. For example, thickness of bedding cushions decreases in the course of use, or vehicle cushions vary in thickness or hardness with extended use. Particularly in vehicle cushions, long time vehicle driving decreases designed thickness and hardness of cushions, lowers prescribed position of the driver, narrows field of view of the driver, impairs safety and damages riding comfort. These problems are originated from the durability of flexible foam and can be evaluated by measuring variation of thickness or hardness in a fatigue by constant-load pounding, and can also be evaluated by measuring variation of thickness or hardness of flexible foam before and after the dynamic durability in vibration condition under load.
Various countermeasures against these problems have been investigated by using dry compression set and wet compression set which are indices of shape stability and durability. The value of compression set can be regarded as the index on evaluation of shape stability of flexible foam. However, in view of practical use condition of flexible foam, the problem is durability in a vibration condition under load. Thus, compression set or other static test alone is unsatisfactory to the index of durability.
Formulation of an increased amount of a crosslinking agent is a practical method for improving the compression set property of flexible foam. However, such a method has limited the range of improvement because this method markedly reduces mechanical properties such as elongation and tear resistant strength of flexible foam. No example for evaluating the dynamic durability in vibration condition under load has also been found.
On the other hand, potassium hydroxide has been mainly used for the catalyst of preparing polyether polyol. Investigations for improving compression set of flexible foam by exchanging the catalyst have also been carried out.
For example, Japanese Laid-Open Patent HEI 02-115211 has described that impairment of compression set can be reduced by using polyether polyol prepared in the presence of a metal cyanide complex as a catalyst. However, no description has been found on the durability improvement of flexible foam. Japanese Laid-Open Patent HEI 03-068620 has also improved wet compression set by means of a similar technique.
However, no description has been found on the improvement of durability. Further, Japanese Laid-Open Patent HEI 03-014812 is the same as above.
Japanese Laid-Open Patent HEI 07-278289 has described that wet compression set can be improved by using polyether polyol prepared in the presence of cesium hydroxide and/or rubidium hydroxide as the catalyst. However, no description has been found on the dynamic durability in vibration condition under load. Moreover, diethyl zinc, iron chloride, metallic porphyrin and metal cyanide complex are very expensive catalysts and raise production cost of polyether polyol. In addition, it is a great problem that, when the resulting polyether polyol is used for the preparation of flexible foam, blowing stability of foam is reduced to render the fresh foam structure unstable and thus processing ability is impaired. Cesium hydroxide and rubidium hydroxide are also expensive catalysts. Although catalytic activity is higher than potassium hydroxide, these two catalysts are still unsatisfactory to remarkably reduce manufacturing time of polyether polyol and thus are difficult to reduce production cost and greatly improve properties of flexible foam.
Consequently, it is now desired to greatly improve durability of flexible foam and to complete a low cost flexible foam having no problem on the processing ability and a preparation process of such flexible foam.